Gear unit combinations often use planetary gear units. Planetary gear units are known, for example, from DE 10 2009 032 286 A1, where the disclosed planetary gear unit has two sun gears, each of which is in meshing engagement with a set of planet gears, which are carried by a planet gear carrier. In this case, the sun gears are radially centered by means of the tooth forces, which in the normal operating mode act in the course of engaging with the planet gears. The drawback with this configuration in that the centering is not guaranteed or is only unsatisfactorily guaranteed under no-load condition, so that the position of the sun gears can tail very far from the radial center position. In addition, imbalances at the sun gears can occur in the normal operating mode despite the acting tooth forces. In addition, alignment errors between the sun gear and the planet carrier are possible. All of these factors lead to a higher degree of wear and an increase in the noise generation. This is especially true, if large forces are transmitted between the planet gears and the sun gears, such as, in particular, in the case of a differential.
Similar gear unit combinations are known, for example, from DE 10 2008 027 992 A1, which discloses a drive device that is intended for motor vehicles having four wheel drive and is provided for transverse installation. In the drive device an axle differential and an interaxle differential are designed in such a way that they are structurally combined in a transmission housing. In this case, the differentials are designed as planetary spur gears, including a first bar as the input element of the interaxle differential, the bar driving the sun gear as the one output element by means of the planet gears and driving the axle differential as the other output element by means of an external gear. In this case the external gear drives the output elements to the axle shafts by means of the bar of the axle differential and the planet gears thereof. This publication discloses that the two differentials are designed in terms of gear technology in such a way that they have a common bar. Hence, a spur gear differential and a planetary gear unit are combined as a superposition stage. However, the torque is tranferred from a front axle to a rear axle of the motor vehicle by way of the superposition stage.
An additional planetary gear unit is known from U.S. Pat. No. 4,574,658. However, in this case a planetary gear unit is combined with a spur gear superposition stage. A sun gear of the planetary gear unit is driven by an additional spur gear section by a pinion that is separate from the planetary gear unit.
The publications DE 2031654 A1 and GB 1212630 A also disclose gear unit combinations, in which different gear stages are connected to each other. These publications disclose, among other things, in particular, a rotationally rigid connection that constitutes a gear stage. The torsionally rigid connection includes two interacting gear elements, each of which carries a ring element that has an axially extending, cylindrical extension and that produces a rotationally rigid coupling with the ring element of the other gear element. In this case the ring elements are made of sheet metal and have in their cylindrical extension axially extending tooth-like indentations, which are in mutual engagement in a partial gear tooth system.
That planet carriers can also be welded together is also known from the publications DE 103 33 880 A1 and DE 103 33 879 A1.
A similar transfer case for motor vehicles is also known from DE 10 2007 017 185 B4, which discloses a transfer case for motor vehicles, the transfer case including a driven differential that drives two output shafts by equalizing elements, so that the output torque at the output shafts is variable by a superposition gear, which is formed by planetary gear units and which is drive-connected directly or indirectly to the output shafts, and by a drive motor, which is coupled to the superposition gear, and where the ratio of the superposition gear is designed in such a way that when the output shafts are in synchronization, the drive motor is stopped. In order to improve the redistribution in terms of precision and faster response with respect to the output torque in a structurally optimal design of the superposition gear and the drive motor, DE 10 2007 017 185 B4 discloses at least one torque-reducing epicyclic gear, which interacts with the differential, be disposed upstream of the superposition gear.
However, such planetary gear units and gear unit combinations have the drawback that they consist of a large number of components and require a relatively large amount of installation space. The production is relatively expensive, and the assembly is relatively complex.